Flexible displays such as “electronic paper” may be formed with electrophoretic materials. Compared to LCD, plasma or other similar flat display devices, electrophoretic display devices have the characteristic that they retain their images after power is turned off. Thus, since power needs to be supplied to an electrophoretic display device only when switching the displayed image, such devices may be used for electronic paper that needs to retain an image over a prolonged time period.
FIG. 1A is a plan view of a unit pixel of an electrophoretic display device. FIG. 1B is a sectional view taken along a line I-I′ in FIG. 1A. A plurality of gate lines 11 and data lines 12 are disposed to intersect on a substrate 10, thereby forming a pixel region on the substrate 10. In addition, common lines 13 are formed in parallel with the gate line 11 on the substrate 10. The common lines 13 are spaced apart from the gate lines 11 by a predetermined interval.
A thin film transistor Tr is disposed within the pixel region. The thin film transistor Tr includes a gate electrode 21 electrically connected to the gate lines 11, an active layer 23 insulated from the gate electrode 21, a source electrode 24a electrically contacted on the active layer 23 and electrically connected with the data lines 12, and a drain electrode 24b electrically contacted on the active layer 23 and spaced apart from the source electrode 24a. 
A capacitor lower electrode 14 electrically connected to the common lines 13 and a capacitor upper electrode 15 electrically connected to the drain electrode 24b are formed within the pixel region. The capacitor upper electrode 14 is overlapped with the capacitor lower electrode 15. An insulating layer is interposed between the capacitor upper electrode 15 and the capacitor lower electrode 14 to form a storage capacitance there between. A first electrode 50 is electrically connected to the drain electrode 24b of the thin film transistor Tr.
An electrophoretic film 60 and a second electrode 70 are disposed on the first electrode 50. The electrophoretic film 60 includes electronic ink that moves by means of electric fields, and a polymer.
The electronic ink formed in the electrophoretic film 60 is driven by an electric field formed between the first electrode 50 and second electrode lines 70 to display an image from the electrophoretic display device.
In order to act as electronic paper, the electrophoretic display device must retain an image over a prolonged duration, and the electrophoretic display device requires a high voltage holding ratio. The voltage holding ratio is a charging voltage ratio over time of a signal voltage first applied to the electrophoretic display device, and the next signal voltage applied thereto. The voltage holding ratio is affected by the surface resistance of the electrophoretic film 60.
Table 1 below shows simulation results of voltage holding ratios parametric in the surface resistances of the electrophoretic film 60.
TABLE 1Surface Resistance (Ω/μm2)1091010101110151021Voltage Holding Ratio (%)6.0875.49799.799.7As the surface resistance of the electrophoretic film 60 increases, the voltage holding ratio of the electrophoretic display device also increases.
When the contacting area between the surface of the electrophoretic film 60 and the conductive first electrode 50 increases, the surface resistance of the electrophoretic film 60 decreases, and the voltage holding ratio of the electrophoretic display device also decreases. When the voltage holding ratio of the electrophoretic display device is thus lowered, the image on the electrophoretic display device may fade. Moreover, when the surface resistance of the electrophoretic film 60 is reduced, an increase of leakage voltage at the interface between the electrophoretic films 60 and the first electrode 50 results.